1. Field of Invention
This invention relates to a bi-directionally current driving circuit, and more particularly to a bi-directionally current driving circuit utilizing a H-bridge configuration, which uses a single power source.
2. Description of Related Arts
For a load element driven by a bi-directionally current driving circuit such as a DC motor, a thermal electric cooler . . . etc, the direction of the current flow will change its physics characteristics. For example, if the current flows from a positive terminal of a DC motor to a negative terminal of the DC motor, the motor rotates clockwise; on the contrary, if the current flows from the negative terminal of the DC motor to the positive terminal of the DC motor, then the motor rotates counterclockwise. Another example is a thermal electric cooler; if the current flows from a positive terminal of the thermal electric cooler to a negative terminal of the thermal electric cooler, then one surface of the thermal electric cooler represents a hot contact surface and the other surface of the thermal electric cooler represents a cold contact surface. If the current direction is reversed, the hot contact surface and cold contact surface will be exchanged too. Therefore, there must be bi-directionally current driving circuits to be developed for these bi-directional applications.
Traditionally, the bi-directionally current driving circuits can be categorized into the following two groups:
Dual Power Supplies Mode
As shown in FIG. 1, a positive power supply Vcc is connected to the collector of an NPN power transistor 101, and a negative power supply VEE is connected to the collector of another PNP power transistor 102. A load element 103 is connected to the emitters of the NPN power transistor 101 and the PNP power transistor 102. The other end of the load element is connected to ground.
A controller 104 drives the NPN power transistor 101 and the PNP transistor 102 based on its input, and then the NPN power transistor 101 and the PNP power transistor 102 provide the needed power to the load element 103. The current direction flowing through the load element 103 depends on the input of the controller 104, which can be either supplied by the positive power source Vcc to the load element 103, or by the negative power source VEE to the load element 103. The mode has the following drawbacks:
1. The mode needs 2 independent voltage sources (positive power source Vcc and negative source VEE) to provide the needed current polarity changes for the load element 103.
2. Power utilization rate is low; the maximum voltage of the load element 103 swings VRL,Max=±(Vi−Vbe). The Vbe for a current power transistor is approximately 0.7 V, which is not an ideal solution for low voltage operations.
H-Bridge with Single Power Supply Mode
The mode consists of four power transistors and a load element, whose geometrical shape is like the capital ‘H’; therefore it is called H-bridge. Based on the configurations of the transistors connection, the mode can be divided into the MOSFET (Metal Oxide Semiconductor Field-Effect Transistor) configuration as shown in FIG. 2 and the BJT (Bipolar Junction Transistor) configuration as shown in FIG. 3. In FIG. 2 or 3, the load elements 205 and 305 are respectively connected to the source terminals of four MOSFETs and the emitter terminals of four BJTs. Its drawback is the low power utilization: the maximum swing in voltage VRL,Max=±(Vi−2 VGS) or VRL,Max=±(Vi−2 Vbe), where VGS and Vbe are the voltage drops across the gate terminal and the source terminal of the MOSFET or the base terminal and the emitter terminal of the BJT.
The H-bridge circuit can be modified to the MOSFET structure (or the BJT structure) as shown in FIG. 4, where the load element is connected to the drain terminals of four MOSFETs 401, 402, 403 and 404, or the collector terminals of four BJTs. With the configuration, the maximum voltage swinging at the terminal of the load element can reach the ideal VRL,Max=Vcc, but the gates and the bases of the four transistors 401, 402, 403 and 404 must be controlled individually. The prior arts still have the following drawbacks:                1. Because the DC control voltages on the individual gates of transistors 401, 402, 403 and 404 are different, they cannot be connected to the gates or bases directly as shown in FIG. 2 or FIG. 3; therefore, the control circuits must be designed individually, which are relatively complicated.        2. If not appropriately designed, the control circuit could possibly produce the currents in vertical direction Iver1 and Iver2. The current originates from power source Vcc, flows through the MOSFETs 401, 402 (or 403, 404), and then arrives to ground. Because the powers are applied to the MOSFETs 401, 402 (or 403, 404) directly, these transistors are easily damaged.        